Radiation-emitting devices are well known and used for radiation therapy or diagnostics. Typically, a radiation therapy device includes a gantry which can be swiveled around a horizontal axis of rotation in the course of a therapeutic treatment. A linear accelerator is located in the gantry for generating a high-energy radiation beam for therapy. This high radiation beam can be an electron radiation or photon (X-ray) beam. Other radiation beam sources can be used as well. During treatment, the radiation beam is typically directed at the isocenter of gantry rotation.
The goal of radiation treatment planning is to maximize the dose to the target volume while protecting radiation sensitive healthy tissue. The X-ray bean intensity often varies over the treatment field by placing an X-ray absorber in the beam path. This allows the target volume to be placed in regions of high beam intensity, while the surrounding radiation sensitive tissue is protected by placement in low intensity regions.
One known device for beam modulation is a wedge (wedge-shaped absorber), used to shape the dose distribution from external photon beams, for example. It is available on the radiation therapy machines of all major manufacturers. The most basic form of wedge is the physical wedge, made of metals such as lead or stainless steel. The physical wedge is placed in the beam path and exponentially decreases the beam intensity laterally across the treatment field. The “toe” of the wedge (i.e., where the thickness of the wedge is the smallest) produces a high beam intensity region, since this portion of the beam has the least attenuation.
An external physical wedge is mounted outside the machine head. A set of standard wedge angles, typically 15°, 30°, 45°, and 60° are exchangeable. A single internal wedge of 60°, called the ‘universal’ wedge, is also used: the wedge is mounted inside the machine head and wedge angles less than 60° are obtained by combining a 60° wedge field and an open field with weights determined by the desired wedge angle. For example, a 30° equivalent wedge is obtained by irradiating half the time with the 60° wedge and half the time with an open field. Since positioning the wedge in place is slow, the beam is turned off during the wedge motion. The movements of a wedge into in-beam position and subsequently into out-of-beam position are in opposite directions. While the wedge functions properly when stationary, un-compensated radiation would be delivered if radiation is applied during wedge motion.
An ‘Omni’ wedge implements wedge orientation by combining weighted orthogonal wedged fields. A ‘Super-Omni’ wedge implements wedging of desired angle and orientation by combining weighted ‘Omni’ wedge and an open field.
The physical wedge has some disadvantages, however. The primary beam intensity is reduced at the target volume; thus, treatment times are increased. Further, scattering of the beam outside the treatment field causes an additional dose to be delivered outside the target volume. It also introduces a spatial energy dependence (i.e., hardness) to the beam, affecting the depth at which the radiation is absorbed across the treatment field. Additional time and effort are required to design, validate, manufacture, install/remove, and store the accessories. In addition, only a limited number of wedge angles are available.
Non-physical wedging is implemented by moving a uniformly attenuating object, e.g., a collimator jaw, across the field in controlled speed and dose rate determining the wedge angle. Non-physical ‘Super Omni’ wedge may produce wedging of desired angle and orientation by using an arrangement of four movable jaws and an open field, whereas the respective fields are properly weighted.
Implementing combinations of sequentially-irradiated fields may be slow and cumbersome. For example, typical jaws speed is in the range of several cm/sec. The time required for a jaw to cover a field-width may be on the order of several seconds.
Arc treatment is an irradiation method where the orientation of the target to the radiation source varies continuously during irradiation. An arc treatment field involves, inter alia, a treatment delivered by continuous rotation of a radiation beam gantry through an angular arc segment while radiation is being applied. Beam aperture and intensity level may be modified for each arc segment. A typical arc segment is on the order of several degrees and the associated time increment is on the order of a second.